1. Field of the Invention
This invention relates to a process for forming a layer of aluminum and tin oxides or aluminum and titanium oxides on a glass substrate. This invention further relates to a glass substrate carrying this layer as well as the use of this layer with a thin, semiconductive, transparent metal oxide layer for use in buildings or in motor vehicles.
2. Description of the Prior Art
Glazings intended for use in buildings usually comprise clear soda-lime-silica glass which exhibits high light and energy transmission factors. For example, soda-lime-silica glass which is 4 mm thick will absorb only about 10% of the light and energy to which it is exposed. To improve the comfort of the building's occupants, particularly in the winter, it is known to cover a face of a glass sheet with a transparent, semiconductive, low emissivity metal oxide layer to increase the rate of infrared reflection of the glazing. Such a layer will reduce the energy loss of the building due to the escape of heat from the inside of the building to the outside through the glazing.
Glazings which carry transparent coatings and exhibit properties of low emissivity are known in the prior art. Some of these glazings comprise, for example, a glass substrate and a thin metal oxide layer of tin oxide doped with fluorine, or a layer of indium oxide doped with tin (ITO). These semiconductive layers may be obtained by various processes, which include the vacuum processes of thermal evaporation or cathode sputtering. Alternatively, these semiconductive layers may be obtained by pyrolysis of metal compounds in the form of a powder or vapor solution sprayed on a heated substrate. The heat of the substrate causes the compounds to decompose and oxidize to form a semiconductive metal oxide layer. These layers exhibit properties satisfactory for use in low-emissivity glazings in buildings.
These semiconductive layers also have optical and electrical properties that make them suitable for use as heated windows and windshields in motor vehicles. Further, these layers may be used in photoelectronic devices such as photovoltaic cells and display devices with liquid crystals.
However, semiconductive layers appear colored in reflection at the thicknesses necessary for obtaining advantageous electronic properties. For example, layers of tin oxide doped with fluorine and ITO layers which are 180 nm thick appear to be blue in reflection, and layers having thicknesses of 360 nm appear to be green in reflection. These reflected colors are often undesirable for a particular application of a glazing. Further, slight variations of thicknesses in the semiconductive layers cause undesirable color irregularities or iridescence.
To eliminate or reduce this color irregularity or iridescence, it has been proposed to deposit on the glass substrate, before forming the semiconductive layer, a so-called intermediate layer or sublayer. The thickness and index of refraction of this intermediate layer is such that the combination of the intermediate and semiconductive layers forms a coating structure that displays a neutral color in reflection.
Intermediate layers that exhibit an index of refraction suitable for forming such a coating structure are, for example, formed from metal oxides, metal nitrides or combinations thereof. These intermediate layers may comprise, for example, aluminum oxide, silicon oxycarbide or silicon oxynitride, or aluminum oxide combined with other oxides such as SnO.sub.2, ZnO, In.sub.2 O.sub.3, TiO.sub.2, etc.
These intermediate layers, with their specific thicknesses and indices of refraction, make it possible to eliminate or significantly reduce the color observed in reflection of the semiconductive layer. However, although iridescence of the semiconductive layer is somewhat reduced, iridescence can still be observed as approximately parallel bands which resemble colored undulations. Although the bands are not as readily noticeable as the bands in a glazing that does comprise an intermediate layer between the glass substrate and the semiconductive layer, these undulations are unacceptable for a product of high quality.